Subject: RE: [IBIS-Users]DC Sweep for the pullup IV curve
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Mon Aug 18 2003 - 09:40:03 PDT
Allen,
OK, there is no pulldown, because the external resistor pulls the
signal to aero volts. But what is that "current source" made out
of? The designs I have seen so far implement current sources with
transistors, except they are sized and biased in such a way that
their IV curve will look more like a current source.
What does a current source's IV curve look like? By definition a
current source provides a constant current regardless of what the
voltage across its terminals is, i.e. the IV curve is a horizontal
line. In theory this should extend to all voltages, however, in
real life (unless it is a specialized circuit design) it will be
polarized (i.e. work only for one voltage polarity, say positive).
So implementing a current source can be done by a huge transistor,
which has a very steep linear region (coming out of the origin),
and then using a fairly low bias voltage you can operate in its
saturation region, which looks like a constant current source.
Note, however, that this transistor is still connected to some
sort of a supply rail.
So regarding your buffer, I would guess that it has this kind of
a pullup transistor, and whether the designer wants to call it a
pullup or not is a different question.
Having said all this, you can make an IBIS model of this buffer
using the "open source" model type, which has a pullup, but no
pulldown IV curve. Don't worry about it being a current source,
the shape of the IV curve will describe that very well.
Regarding clamps, I have a hard time to believe that the above
transistor doesn't have any parasitic diodes (unless it is not
MOSFET) and that there is absolutely no ESD protection in this
device. It may be the case, but usually it is not. So I would
expect that you would still see some currents outside the rails
when you 3-state the device for this reason. These currents
will go into the IBIS clamp IV tables.
Arpad Muranyi
Intel Corporation
=================================================================
-----Original Message-----
From: Allen Chao [mailto:achao@synopsys.com]
Sent: Monday, August 18, 2003 8:04 AM
To: Muranyi, Arpad; ibis-users@eda.org
Subject: RE: [IBIS-Users]DC Sweep for the pullup IV curve
Hi Arpad,
Thanks for your fast reply... I just consulted further with the buffer
designer here. He said that this output is a high-speed data output and
instead of using Vcc (power-supply voltage) for the power clamp diode and
the pull-up transistor, we use a current source to drive the output to 0.4v
for signal high and simply turn off the transistors between current source
and the output for the low signal. (since the output is also connected to
another resistor and ground, the output will be grounded for the low
signal). As I described, there are not really pullup/pulldown transistors
and clamping devices.
My question is, can we still create an IBIS model for this output?
Thanks in advance,
Allen Chao
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